Fiber optics are typically constructed as either basic UV coated fibers (typically 250 micron diameter) or tight buffer coated fiber (typically 900 micron outer diameter). Tight buffer optical fibers are used in many cases where a stand alone optical fiber is desired. However, when a UV coated fiber is desired they are typically loosely bundled into a common tube for installation and handling. This “loose tube” arrangement typically has 12 UV coated fiber optic elements surrounded by a loosely fitting extruded polymer tube. More or less UV coated fiber optic elements may be used as well, but twelve fibers per tube is a common configuration.
In order to meet construction requirements, many additional features and elements may be added to the tubes. For example, to prevent water ingress, gels were commonly used, applied to the fibers prior to the tube extrusion to prevent water from entering the tube in the case that the tube is breached due to environmental conditions. However such gels are difficult and expensive to use during cable creation, and also difficult to use during applications where the gel needs to be cleared away before splicing operations. Additionally, although the fibers are technically loose with the tube, the gels effectively couple the fibers to the tube wall, and thus pressures and tensions applied to the tubes during installation may be inadvertently carried into the fibers themselves, with gel further acting to lock such strain into the fibers.
To overcome this in the prior art certain loose tube arrangements have been fabricated to be gel-free or “dry.” In one arrangement, water swellable powders are used as water blocking agents. However, as more water swellable powders are added within the tube, this can cause micro-bending at the locations where the powder particles are wedged between the inner surface of the tube and fibers. See prior art FIG. 1.
Moreover, certain polymers used to form the tubes, such as PVDF (Polyvinylidene Fluoride—used for its flame and smoke effectiveness), exhibit increased variability in their post extrusion shrinkage and can cause the length differential between the tubes and the fibers to be excessive, a condition that is detrimental or even fatal to the cable's effectiveness. This condition may be exacerbated in dry tubes that lack the gel compound which would have tended to moderate such excessive post extrusion shrinkage.